1. Field of the Invention
This invention relates to an immunologically active protein, and the gene which encodes it, useful in vaccination against malaria. More particularly, this invention relates to a protein antigen on the surface of Plasmodium yoelii sporozoites detected by a monoclonal antibody NYS4, clones of genomic and complementary DNA sequences specific for this and closely related proteins from malarial parasites which, when administered as vaccine components either alone or in combination with other malarial antigens, can confer protective immunity.
2. Description of the Prior Art
Prevention of infection by human malarias would alleviate a major health problem in the tropical and sub-tropical areas of the world. The most promising method for the control of malaria appears to be the development and use of vaccines.
Species of the genus Plasmodium are the etiological agents of malaria. These protozoan parasites have a complex life cycle involving reproduction in mammalian hosts and insect vectors. The infectious stage of the parasite is called the sporozoite. It is inoculated into a mammalian host by the bite of infected Anopheline mosquitos.
One approach to producing a malaria vaccine is to attempt to induce an immune response against the infective sporozoites themselves. This has been done by immunization of human and animal models with radiation attenuated sporozoites. These attenuated sporozoites consistently protect against challenge with infectious sporozoites (Nussenzweig et al., Nature 216:160, 1969, Clyde et al, Am. J. Med. Sci. 266:160, 1973, Rieckmann et al., Bull. W. H. O. 57, Suppl.1:261, 1979).
A surface protein of Plasmodium sporozoites, designated the circumsporozoite protein (CSP) has been identified and well characterized (Nussenzweig and Nussenzweig, Cell 42:401, 1985). The CSPs of several Plasmodium species have been described. They all contain tandem repeats of short peptide sequences as well as regions of non-repetitive sequence. While the existence of tandem repeats is a conserved feature of all CSP genes, the particular amino acid sequences which are repeated vary both between and within species. On the other hand, the non-repetitive sequences are well conserved both within and between species.
During the past decade, the primary strategy for malaria sporozoite vaccine development has been to produce vaccines that induce antibodies to the repeat region of the CSP. These antibodies are believed to prevent effective sporozoite invasion of hepatocytes (Mazier et al., Science 231:156, 1986, Young et al., Science 228:958,1985, Ballou et al., Science 228:996, 1985, Zavala, et al. Science 228: 1985, 1985). Thus far, protective immunity after immunization of humans (Ballou et al., Lancet 1:1277, 1987, Herrington et al, Nature 328:257, 1987) and non human primates with CSP based vaccines (Collins, et al. Am. J. Trop. Med. Hyg. 40:455, 1989) has been disappointing. Mice immunized with P. berghei subunit vaccines based on the CSP have been protected against moderate, but not against large challenge doses of sporozoites (Egan et al. Science 236:453, 1987, Zavala et al. J. Ex. Med. 166:1591, 1987, Hoffman et al., J. Immunol. 142: 3581, 1989, Romero et al. Eur. J. Immunol. 18: 1951, 1988). In the P. yoelii model system, mice have been immunized with a variety of synthetic peptides and recombinant proteins based on the CSP (Lal et al. Proc. Natl. Acad. Sci. USA 84:8647, 1987, Sedegah et al. in Technological Advances in Vaccine Development L. Lasky editor, New York, 1988, Sedegah et al., Bull. W. H. O., in press, Charoenvit et al., Bull. W. H. O., in press). In the majority of experiments, mice developed high levels of antibodies to the CSP, but were not consistently protected against challenge with P. yoelii sporozoites.
The failure of immunization with vaccines based on the CSP to provide the same solid immunity as immunization with radiation attenuated sporozoites suggests that although the CSP plays a role in immunity, there are other sporozoite antigens important in immunity to malaria. There is a need to identify, isolate and characterize these antigens so that they may be included in future malaria vaccines.